... when the last film ended with Frodo and Bilbo sailing off from the Grey Havens. There's no concievable way they can follow on from that. If this is right and Frodo is to be in the opening sequence of the first Hobbit film we still don't know what the context will be. Will Bilbo be telling him the story before the book is written (so setting the scene some time before the action of Fellowship of the Ring). Or will they be in Rivendell? Or will Frodo be reading from the book after his own adventure but before he leaves Middle Earth? Or have they come up with some completely different scenario? Or will they change their minds about how to begin the film several times between now and 2012, so that what they're intending now might not happen at all in the final film?

The one thing The Hobbit cannot be is a sequel to Return of the King (unless we join them on the deck of the ship or in Aman!)